Frequency control of an oscillator of the velocity modulation type



Jan. 13, 1948. H. M. sTEARNs FREQUENCY CONTROL O1"V AN OSCILLATOR OF THE VELOCITY yMODULATION TYPE Filed May 11, 1943 4 Sheets-Sheet l T H www man o www f `H. M. srEARNs 2.434.293 FREQUENCY CONTROL 0F AN OSOILLATOR OF THE VELOCITY MODULATION TYPE l Filed May 1l, '1943 4 Sheets-Sheet 2 Jan. H3, 1948.

H. M. sTEARNs 2,434,293 FREQUENCY CONTROL OF AN OSCILLATOR OF THE VELOCITY MODULATION TYPE Jan. 13, Y194s..

4 Sheets-Sheeft 3v Filed May l1, 1943 INVENTOR Hak/)c5 BY Jan. 13, 1948. H M- STEARNS l2,434,293

FREQUENCY CONTROL OF AN OSCILUATOR 0F THE VELOCITY NWLAZION TYPE Filed may 11, 1943 4 Smets-sheet 4 mvENToR l/aecf Mme/ STEAK/vs ATTORNEY Patented Jan. 13,1948

. t, FREQUENCY CONTROL OF AN OSCILLATOR OF THE VELOCITY MODULATION TYPE Horace Myrl Stearns, Merrick, N. Y., assigner to Sperry Gyroscope Company, Inc., a corporation of New York lApplication May 11., 1943, Serial No. 486,589

25 Claims. (Cl. Z50-36) The present invention is related to the art concerning automatic frequency control systems, and especially, but not restricted to, those adapted to maintain proper frequency output of velocitymodulation type oscillators. y

The present system is especially adapted for th control of a velocity modulation oscillator of the so-called reilex type, such as disclosed' in Fig. 2 of Varian and 'Hansen Patent No. 2,250,511, issued July 29, 1941, but is not limited thereto. In such an oscillator, an electron beam is passed through a hollow cavity resonator and is thereafter reflected by a suitable reflector electrode and re-enters the resonator. The rst passage of the beam through the resonator produces periodic variation of the electrons of the beam. These electrons, during the ensuing period in which they travel toward the reflector and are reflected therefrom, become grouped or bunched and impart ultra-high-frequency energy to the resonator upon their reentry therein, whereby ultra-high-frequency oscillations are set up and maintained in the cavity resonator. The frequency of the output oscillations of such a device may be adjusted within narrow limits by adjusting the electron transit time by adjusting the potential applied to the reflector electrode. This frequency may also be varied over wider limits by adjusting the resonant frequency of the cavity resonator, as by varying the physical dimensions or grid spacing thereof. One method of thus tuning a reflex velocity modulation oscillator is that, after a suitable period, the oscillator will have drifted to the proper operating frequency. This, however, is open to the objection that a considerable time may be necessary for warm-up. Other attempts to overcome this problem have involved the use of special thermal compensation to reduce the frequency drift due to warm-up. Such devices'however, have been extremely complex and have only approximately attained the desired result.

According to the present invention, an oscillator is operated so as to automatically seek out and maintain its output frequency in fixed predetermined relationship to a standard frequency, which, in the illustration to be described, may be a received wave with which it is desired to maintain the oscillator output in fixed relation, such as with a fixed frequency difference therebetween.

' ing or scanning `action is automatically intershown in copending application Serial No. 462,- A 436, filed October 1'?, 1942,.-inthe names of D. L.

Snow et al. Asis shown in this copending appliis controlled byathermally expansible elementwhose expansion is controlled by current passingtherethrough, which heatst'he expansible element and thereby causes extension fthereof. Thel present invention. is particularly .adapted to control cation; tnituningof the resgnator of. thecavity maintain 'a predetermined-frequency relationship' oscillatorsofgthe type shown-,zin thiscopending alpplicationy,butv it isr to bel understoodthat the g pres'nt invention isV not-necessarily .restriizteri-l to# such oscillators.

In electronic apparatus, generally, the device does not attain its stable operating condition immediately, but requires a warm-up period during which the various parts attain their operating temperature and warmed-up condition. During this warm-up period, the output frequency of oscillators, in particular, drifts widely. One method of compensating for this drift is to so select the initial energizing conditions for the oscillator vention to maintain the output frequency of an oscillation generator in fixed predetermined relation to any' suitable source of reference frequency. Itis another-object of the present invention to between an arbitrary reference frequency and thev output of Va reflex velocity -modulatio'n oscillator. A

Itis yet another object of the present invention to control 'the output frequency of. an oscillator to automatically-'hunt for its desired output frequency, and to automatically maintain that desired -output frequency l.when-it has heenonce attained.

A further object of the present invention is to provide improved apparatus for sweeping the output frequency of an ultra-high frequency oscilla- It is yet a. further object of the present invention to provide improved control of reflex velocity modulation electron discharge devices by simultaneous control of the reector electrode and tuned circuit thereof.

It is still another object of the present invention to provide improved control of a thermally tuned oscillator, to sweep its output frequency across a desired and adjustable band,` and to maintain its output frequency at adesired value.

Other objects and advantages of the present invention will become apparent from consideration of the following drawings and specification, wherein,

yFig. 1 shows a schematic block diagram of one type of system to which the present invention has particular application:

Fig. 2 shows a wiring diagram of the present invention, including those parts of Fig. 1 Within the dot-dash line 22 thereof;

Fig. 3 shows voltage-time graphs useful in explainin'g the operation of the device of Fig. 2;

Fig. 4 shows the voltage-frequency characteristic of a conventional frequency discriminator, as used in Fig. 2;

Fig. 5 shows magnied versions of portions ci the voltage-time graphs of Fig. 3;

Fig. 6 shows a circuit diagram of a modication of the device of Fig. 2, being alternatively usable-in place of the portion of Fig. 2 to the left of line A--A:

Fig.7 shows a voltage-time graph similar to that of Fig. 3, useful -in explaining the operation of the device of Fig. 7; 1

Fig. 8 shows a circuit diagram of a modification of a portion of the devices of Figs. 2 or 6, being alternatively usable in place 'of the portion of those gures to the right of line A-IA.

Fig. 9 shows a schematic diagram of an alternate form of oscillator which may be substituted for that shown in Figs. 2.or 8. f

Fig. 10 shows a schematic diagram of still another form of oscillator useful in the circuits'of Figs. 2 and 8, but using magnetic tuning; and

Fig. 11 shows another type of system in which the present invention may be used.

Considering Fig. 1, there is shown, for illustrative purposes, a system'in which the present invention may be utilized. It is to be understood, however, that the present invention may also be utilized in many other and widely different systems, the present system being shown for illustrative purposes only. The system of Fig. 1 is a radio object detecting and locating system in which a pulse transmitter II produces a sequence of periodically repeated pulses of ultrahigh-frequency energy which are radiated direcject. This intermediate frequency then amplifled in a suitable conventional intermediate frequency amplifier I'I, whose output is connectdone,-as in the present case, by suitably controlling the local oscillator I6 to maintain it in fixed frequency relation with respect to the wave received by antenna I3; for example, having a fixed frequency dierence with respect to the frequency of the received wave. For this purpose, the output of intermediate frequency amplier I'I is fed to a frequency discriminator circuit I9 of any conventional type which is adapted, as is well known, to produce output potentials corresponding to the deviation of the intermediate frequency supplied to it from a predetermined frequency to which the discriminator isrtuned, these discriminator output potentials having a predetermined polarity for one sense of such frequency vdeviation and having opposite polarity for vopposite sense of frequency deviation. Fig. 4 shows the usual output voltage versus input frequency characteristic for such a disv' criminator, the discriminator being fd.

In the present instance, these potentials derived from discriminator vIii operate to revise the output frequency of oscillator I6 to maintain the intermediate frequencyderived from mixer I4 at substantially the value fd for which the discrimtuned frequency inator is tuned. This control of oscillator I6 is produced by a control circuit 2| responsive to the output of discriminator I9.

`It will be noted that, since the Wave transmitted by transmitter II and reflected by the distant object is a periodic pulse wave, the output of mixer I4 will be a sequence of pulses of theintermediate frequency, and like wise the output of discriminator I9 will be a sequence of uni-directional voltage pulses, but having a polarity and magnitude corresponding to the sense and magnitude of the frequency deviation of the intermediate frequency from the .desired value as deter-- mined by the tuned frequency fd of the discriminator I9. 'The present invention is directed more particularly to the details of the control circuit 2| in its relation to the local oscillator I6, as will be seen in Fig. 2, which shows the circuit diagram of the portion of the system of Fig. 1

A contained within dot-dash line 22|, that is,. the

tionally by a suitable directional antenna I2 toward the distant object to be located. When such an object is irradiated by this ultra-highfrequency energy, a portion of the energy inciv dent on the object will be reflected thereby, and

ving a value which is the difference between the frequencies of the wave producedy by local oscillator I6 and that received from the distant obcontrol ci'cuit'Z'I and' t-l local oscillator I6.

vAs is seen in Fig. 2. the local oscillator I6l I therein diagrammatically illustrated is of the thermally tuned reflex velocity modulation type, and comprises a cathode 23 maintained ata high negative potential with respect to the casing of a resonator 26 by means of a. battery 24, so that" a beam of electrons is projected from cathode 23... throu'ghl the grids 21 of resonator 26. A 'reflector electrode 28 is located in the path of the electron beam leaving the resonator 26, and has applied thereto a potential negative with respect to the resonator 26, which potential may be slightly positive or slightly negative with respect to cathode 23, as adjusted by meansof a tap 29 onl battery 24.' Reflector 28 is thereby adapted to reverse the electrons of the stream and to reproject them into the resonator 26 to set up and y plete tuning range.

sustain oscillations therein, in accordance with the well-known theory of such reflex oscillators,

- described more in detail withrespe'ct to Fig. 2 of nected by exible diaphragm 30 to permit adjustment of the size of resonator 26 and the relative spacing of grids 21, whereby the output frequency of the oscillator I6 may be adjusted. To

adjust this output frequency, a thermally expansible tuning strut, indicated diagrammatically at 32, is interposed between the two portions ofthe resonator 26. Surrounding the tuning strut 32 is a suitable heating coil or resistor 33. In this way, by passing a predetermined amount of current through heater 33 the tun- I' ing strut 32 will be expanded or contracted by a corresponding predetermined amount, and will adjust the output frequency of the oscillator I6 correspondingly. Atmospheric pressure on the evacuated resonator takes up back-lash, or a tension spring coupling the two parts of resonator 26 may be used. As shown here, an increase in strut current will separate grids 21, to increase the output frequency, but the tube may be arranged, if desired, to decrease its frequency with increase of strut current.

The remainder of the circuit of Fig. 2 represents the control circuit 2| for adjusting the current supplied to theheater coil 33, to make the output frequency of oscillator I6 properly related to the frequency received by mixer I4; that is, to hunt for and to maintain the proper intermediate frequency.

In .order that oscillator I6 may hunt out its required operating frequency, its output frequency is periodically swept or scanned over its'com done by periodically charging-and discharging a large condenser 34. Condenser 34 is connected in series with an adjustable source of negative potential, represented by a voltage divider 36, through a gas discharge or trigger tube 31 which may be of the cold cathode type, commercially known as the OAl-G. Such tubes have a cold cathode 38, a, discharge-initiating or starting electrode 39 and an anode 46. As is well known, when a predetermined ring voltage is impressed between the starting electrode 39 and the cathode 33,

an electric discharge is initiated-between anode 4|) and cathode 38, which will be maintained so long is determined by'ritsv connection to point X of a I voltage divider -formed by-resistors 62, 66---andh42-.-

connected between a negative potential source 43 and a positive potential source 56. Point X generally will have a negative potential with respect to ground. Voltage divider 36 has one end grounded as shown and the other end connected to a further source of negative potential 44, which supplies a larger negative voltage than does source 43.

Let it be assumed that initially the voltage Ve (Fig. 3) of cathode 38, as determined by Voltage In the present-case, this is 'divider 36, is adjusted to be more negative than the potential Vx of point X by an amount exceeding the ring voltage V: of the trigger tube 31i Accordingly, upon energization of the circuit, tube 31 will immediately discharge. and will becomeessentlally a, short-circuit, whereby condenser 34 is rapidly charged up to substantially voltage Vc as shown at 6|). Since anode 40 is connected to the high negative potential side of condenser 34, it will be seen that its potential is thereby reduced to a value substantially equal to that of cathode 38. For this reason. the discharge between cathode 38 and anode 40 is interrupted, and an essen# v, tially open circuit is interposed between leenden-- ser 34 and voltage divider 36. Condenser 34 now 1 proceeds to discharge, through a resistor 4| connected in parallel thereto, at a rate which may be controlled by suitably adjusting or selecting the resistance of this resistor 4|, which may be made variable as shown. As the charge of condenser 34 leaks oil? through resistor 4|, its voltage gradually decreases, as shown by curve of Fig. 3. The potential of anode 40 and" starting electrode 39. to which condenser 34 is connected,

also followcurve 45.

When the negative potential of starting electrode 39 with respect to ground reaches the value Vs diering from that of cathode 38 by an amount equalling or just exceeding the firing potential Vr, trigger tube 31 will again discharge, thereby-v recharging condenser 34 and initiating a further similar cycle of operations. In this manner, neglecting for the present the eiect of the remainder of the circuit, the voltage across condenser 34 is' cyclically varied in sawtooth fashion as is shown at 60, 45 in Fig. 3.

This voltage across condenser 34 is connected through a coupling or grid-current limiting resistor 46 to the control grid of a power tube 41 operating as a class C amplifier, so that this voltage serves to control the conductivityvof tube- 41. Superposed on the control grid of tube 41 through a blocking condenser 49 is a suitable alternating voltage supplied through terminals as anode 45 remains positive with respect to,- cathode` 38. Under these conditions, tube31 isI 48 and coupling condenser 49. This alternating voltage has a relatively low frequency, but of a value suiciently high so that the thermal inertia of heater coil 33 will prevent the expansion and contraction of tuning strut 32 from following the alterations of voltage supplied to terminals 48.

As an illustrative example, this low frequency voltage may have a frequency of 1-50 kilocycles.

Preferably, this voltage has a square wave form,

so that the power delivered to strut 32 will vary --linearly with the voltage of condenser 34.Y Tube.

41 acts as a power amplifier and amplifies the low frequency voltage connected to terminals 48.

- The outputl of tube 41 is supplied through an output transformer 5| directly to the heater coil 33.

It will beseen therefore that the low frequency voltage connected to terminals 484 serves to energize the heater coil 33 and thereby heats the tun- ".ing strut 3 2 `and controls the output frequency ofoscillatorv I6' in accordance ,with'the amplitude of-the-outptitlof--poweritube 41.- This out-put is controlled by the voltage derived from condenser 34. As discussed above, this voltage is of sawtooth form as shown at 6D, 45 in Fig. 3 so that the energizing current of tuning strut 32 will have a corresponding sawtooth form, as shown at 50, and

will thereby sweep or scan the output frequency' oi oscillator |6 between two limits which may be selected to be substantially the tuning limits of the oscillator. As shown at. 55 in Fig. 3, the frequency sweepwill lag slightly. behind the con..

' as its output resistor.

denser sawtooth voltage 45 because of the tuning strut thermal inertia.

The frequency of the sawtooth oscillations may be adjusted by adjusting the resistance of resistor 4I to adjust the discharge rate of condenser 34. The scanning limits may be adjusted by ad.. justing the potential Ve of cathode 38 of the 'trigger tube 31 by means of variable voltage divider 36. It will be noted that the connection of anode 40 to point Xprevents the frequency of oscillator I6 from going lower than the value corresponding to Vx. Thus, if Ve is decreased so as to differ from Vx by less than Vs, tube 31 will polarity which. as shown in Fig. 4, is positive. .'-is the mixer output or intermediate frequency passes fa, the polarity of the output of frequency discriminator I9 will reverse, and the discriminator will now yield a negative output. This output of discriminator I9 in each case will be in the nature of periodic unidirectional pulses corresponding to the pulses transmitted by transmitter I I'. This-pulse repetition rate is suffino longer trigger, and no sawtooth oscillations can occur. Hence, the other scanning limit may be adjusted by adjusting the potential of point X. The voltage divider 42, 66, 62 may be made adjustable for this purpose.

In this way, oscillator I6 may be caused to periodically scan its output frequency between desired limits and ata desired rate. Preferably, the frequency of sawtooth oscillations is adjusted to a fairly low value, such as of the order of s to 16 cycles per second, which rate of frequency variation is low enough to permit the output frequency of oscillator I6 to follow the sawtootn scanning, as shown at 55.

Thus far, the manner in which the frequency discriminator I9 controls the oscillator4 I6 has not been described. Briefly, discriminator I9 acts to prevent the discharge of condenser 34 to a value at which the trigger tube 31" will' discharge, so long as the output frequency of oscillator I'3-has the value which gives a proper intermediate frequency value. Thus, the output of discriminator I9 is supplied to an amplifier tube 52 of any conventional type having a conventional output resistor 54 connected to a positive potential source 56. The output of tube 52 in turn is vcoupled to a gas switching control tube 53, such as of the conventional type 2050, by way of a coupling condenser 51. The cathode 56 of control tube 53 is connected to a source of negative potential 59, which may be of the same or a lower voltage with respect to source 43. Anode 6I of tube 53 is connected to positive potential source 56 through a resistor 62 which thereby also serves A suitable bias for the control grid 63 of tube 53 may be provided, as byv a suitable variable voltage divider 64 connected to a source of higher negative potential 55. Control tube 53 4has its bias adjusted so as to be biased slightly beyond cut-01T. In this way, it does not respond in any manner to negative pulses ciently high with respect to the rate at which the frequency of oscillator I6, and hence the intermediate or mixer output frequency. is being swept through the tuning band so that a plurality of these pulses will pass through the discriminator I9 during the time that the intermediate frequency is swept past the tuned frequency fd of discriminator I9. Now, as the intermediate frequency approaches the discriminator frequency fd, the positive pulses thereby produced are amplified by amplier 52, which reverses them in polarity so that corresponding amplified but negative pulses are applied to grid 63 of tube 53. Since the tube 53 is biased beyond cut-off, these negative pulses have no effect.

When the intermediate frequency has passed the discriminator frequency fd, the pulses produced by the discriminator I9 reverse in polarity and now become negative. These negative pulses applied to amplifier 52 are amplified and reversed in polarity, so that amplified positive pulses are impressed on the control grid of tube 53. When these positive pulses exceed the bias on grid 63 (corresponding to an intermediate frequency fi), they cause the gas tube 53 to become co'nductive, drawing current through its output resistor 62, and thereby decreasing the potential of anode 6|. Anode 6I is connected to corresponding decrease in thepotential (or ining the time tube 53 is conducting. This is shown 'l applied to grid 63 but will respond to positive 55 pulses exceeding the bias. The bias is so adjusted that tube 53 will not respond to noise pulses, but only to signal pulses of higher amplitude than the noise pulses. If desired, grid 63 may be biased just at cut-off, so that tube 53 60 will respond'to all positive input pulses.

The operation 'of the device may be explained with reference to Fig'. 4, which shows the relationship between the intermediate frequency and the output of discriminator I9 for'a fixed input voltage to discriminator I9. This is the-wellknown frequency discriminator characteristic.

It will be seen that as the output frequencyja of oscillator I6' is swept through its tuning band, this frequency will approach, attain, and pass a certain desired value, at which the mixer output frequency equals the discriminator frequency fd. As the oscillator frequency ,fo approaches this desired value from a higher value, discriminator I9 will produce an output having a predetermined crease the intermediate frequency above fir-thecrease in negative potential) of anode 40, which is connected to the negatively charged plate of condenser 34. The negative charge on condenser 34 is therefore increased by a slight amount durafter the positive pulse'on grid 63 ceases. As-

soon as tube 53 is blocked, the recharging of condenser 34 is cut off and condenser 34 resumes its ,discharging through resistor 4I, as shown at 1I in Fig. 5. .-f V `f If the recharging of condenser 34 did notre-1 duce the tuning strut current sufficiently to Yirnnext discriminator pulse, which is then still positive of the proper polarity, will again trigger the tube 53 very shortly thereafter, to further recharge condenser 34, as shown at 12 in Fig. 5and the same cycle repeats itself until the intermediate frequency climbs -back above fi. When this happens, the discriminator output pulses are no longer effective to trigger the tube 53. Then condenser 34 is permitted to discharge until the theyhave relatively little effect on lator I6.

i age strut current.

.of condenser 34 or the strut current, but will remain substantially constant or will only vary slowly.

By making the amplification of amplifier 52 and tube 53 relatively large, the frequency characteristic of discriminator I9 shown in Fig. 4 can be made quite steep, so that the intermediate frequency fi at which the system is maintained will differ from the discriminator frequency fd vby a very small amount, having a negligible effect on the operation of the circuit.

By the proper choice of circuit constants, the recharging of condenser 34 in response to a single discriminator pulse vcan be made sufficiently large so that it will take the condenser 34 a period of several discriminator pulses to drift back across the value producing intermediate frequency f1. In this way tube 53 need operate in response to only one out of four or ve discriminator pulses. The voltage of condenser 34 then correspondingly oscillates between two close limits as' shown at l 69 in Fig. 3, at a frequency one-fourth or onefth of the pulse repetition rate. The thermal inertia of the tuning strut prevents a similar oscillation in the oscillator frequency. If, for

' any reason, the discriminator outlet pulses fail for a short time, as for example due to failure ofthe transmitter, the thermal inertia of the strut will maintain almost constant frequency until the pulses resume control.

The switching operation of tube 53 already described occurs very rapidly 4and tends to produce a momentary but relatively high recharging voltage impulse for condenser 34. In order to lengthen the effect of this recharging voltage and to eliminate its. very high amplitude which would necessitate high voltage condensers and insulators, a condenser 61 is shunted` across resistor 66 and serves to by-pass the higher-frequency components of the transient pulse so that the recharging voltage. Y

.'I'he above description was based on thev tacit assumption that the desired operating frequency of oscillator I6 required a predetermined xed strut current correspondingto a predeterminedv setting of the tuning of resonator 26 of oscil- However, it may happen that oscillator I6 has an inherent tendency to drift, in

4 which case resonator 26 must be continuously reset to maintain constant output frequency.

This, of course, requires a change in the aver- The present circuit is inherently adapted to operate properly under these conditions also.

Thus, if the oscillator frequency drifts so as Vte increase the intermediate frequency above the value fi the discriminator pulses will have no effect on tube 53 and condenser 34 then discharges through resistor 4I until the intermediate frequency once more passes below fi. The

V average value of condenser voltage is then mainy tained at the new value required to again produce intermediate frequency fl. This average condenser voltage will be lower than that existing before the drift occurred, as is required by the, nature of the control desired.

On the other hand, should the oscillator frequency drift tol decrease the intermediate frequency f1, more of the discriminator pulses will trigger tube 53, producing a greater recharging eiect for condenser 34, and thereby increasingv as a gas tube which was triggered by the dis- However, the system criminator output pulses. will operate in substantially the same manner if tube 53 is an ordinary vacuum amplier tube,

again biased beyond cut-oi so that random noise pulses will produce no output therefrom. Then, when the Vintermediate frequency drifts below the value fi, the negative discriminator pulses thereby produced will be amplied and reversed by tubes 52 and 53, and will serve to recharge condenser 34 to return the oscillator fre.- quency to the proper value as already described. If the recharging eiect of a single pulse is in-I sufficient to prevent the oscillator from continuing its drifting, the next succeeding pulse will have a larger amplitude, due to the fact that the intermediate frequency has deviated by a larger amount from the 'discriminator frequency fd, and will produce greater recharging. Again by making the amplification of discriminator4 pulses relatively large, the amplitude of the rst or surely the second discriminator pulse occurring after crossing frequency fi may be made sufficiently large to recharge condenser 34 to maintain the oscillator frequency at the desired value. The system will then'operate in exactly the same manner to prevent any change in the intermediate frequency, such as due to a change in the frequency radiated by transmitter II or has shifted beyond the operating range of discriminator I9, the charging control pulses are no longer produced by discriminator I9. Accordingly, condenser 34 will resume its discharging through resistor 4I, permitting trigger tube 31 to fire as already described, so that the oscillator I6 will once more be swept through its tuning range until its frequency approaches the proper operatingv frequency, lock in. Accordingly. the system will automatically regain the desired operating frequency even after it has been lost for anyreason.

Fig. 6 shows a modification of the invention which may be used in place of the portion of the circuit of Fig. 2 to the left of line A-A thereof. In this case, the direction of band sweeping of the oscillator frequency is reversed from that when the system will once more Y 11 shown in the modification of Fig. 2. Thus, in

Fig. 6 the condenser 34', -corresponding to the charging condenser 34 of Fig. 2, is now connected between ground and the cathode 38' of the gas discharge tube 31'. The anode 40' and ringelectrode 39 are connected to the adjustable tap of variable voltage divider 36', 4which is energized from a source of positive potential 56'. The cold cathode 38 of tube 3l' is connected to the point X of the voltage divider comprising resistors d2', 66' and 62', which are connected in series between a source 56. of positive potential and a source 59 of negative potential.

In this manner, when the potential applied to anode d' and starting electrode 39' is made more positive than the potential of cathode 38 and point X by an amount exceeding-the ring voltage of the tube, tube 3l' will discharge, permitting` l2 rent through the resistor 62'. This causes an increase in positive potential of point X, and serves to recharge condenser 34' during the duration oi' the conduction of tube 63'. Condenser 'lil' serves to block tube 53' shortly after the input positive pulseceases. Therefore the action of the apparatus is exactly the same as in Fig. 2, and the voltage oi condenser 34' will vary slightly about the value at which the proper oscillator condenser 34' to be positively charged substan tially to the potential of anode d'. Immediately after discharge, tube 3l' blocks itself and condenser 34' begins to discharge through resistor di'. When condenser 34 has discharged to a value such that the difference in potential between "anode 40' and cathode 38' is again equal to the firing voltage, tube 31 triggers once more and the cycle of operations is repeated. The con-` nection of cathode 38' to point X assures that A the voltage of condenser 38 will never drop below the potential of point X as determined by the voltage divider 82', 66' and 52' across sources 5S and 59'.

It will be seen that the system is substantially identical with that of Fig. 2 with the exception that condenser 36' produces a positive sawtooth control voltage. This voltage across condenser 34' is then led to the power amplier dl as in Fig. 2. However, it will be noted that this power amplifier is in this instance additionally provided with a source of bias (not shown) sumcient to permit proper operation as a class C amplifier,

In this Way, the sawtooth voltage of condenser 34' causes the output frequency of oscillator i6 to sweep across its frequency band periodically as before, but in a reverse sense. This 'voltage across condenser 34" is illustrated in Fig. 7, and is through a coupling condenser 51' and a series grid resistor 'il'. Tube 53' is now operated as a cathode follower tube, having a positive. potential applied directly to its anode 6l' from source 56', 'and having its load or output resistor 62' connected between its cathode and a source 59' of negative potential. Since'the cathode of tube 53' is maintained more positive than' source 59', by virtue of the action of voltage divider 42', 66' and 62', the negative bias on grid-63- may be obtained and adjusted by means of a variable voltage'divider 64 connected between source 59' and ground. This bias is then set so that tube 53 will respond only topositive pulses derived from the discriminator output having an amplitude exceeding the grid bias, which is preferably adjusted to cut off undesired noise voltages. Resistor 'il serves as a grid current-limiting resistor, should the pulses from the discriminator drive grid 63 positive. y

In this manner, when a sufficiently positive pulse is received from the discriminator, tube 53' becomes conductive and increases the potential in Fig. 4, which is greater than the discriminator frequency fd by approximately the same amount as frequency f1 is less than fd. Tube 53' may be a vacuum tube, as discussed above relative to tube ttf Therefore, by choosing either the circuit of Fig. 2 or that of Fig 6, either direction of band sweeping may be produced as desired.

Fig, 8 showsa modification of the portion of Fig. 2 to the right'of -line A--A, which may also be used with Fig. 6,v if desired. In this instance, instead "of controlling the class C amplifier or power amplifier tube 4T from the voltage of condenser 3@ or 34', this same voltage is utilized to control the output of a Alow frequency oscillator 12, having anoutput frequency similar to that connected at 48 of Fig. 2. Oscillator 12 is of the conventional type, but has its plate voltage, de-

rived from source E6, controlled by a control tube through a series resistor 16. The condenser voltage thereby controls the conductivity of control tube it, which in turn controls the plate potential applied to oscillator l2. Oscillator 'l2 is adapted to produce an output wave of a frequency similar to that impressed on terminals 48, the amplitude of this wave depending upon the plate voltage of the oscillator 12.. This output is then supplied to the heater 33 for the tuning strut 32 of oscillator I6, similar to Fig. 2. In this manner an alternative form of control for the vtuning strut 32 is provided. The tuning range is now adjusted by adjusting potentiometer tapvl,

Fig. 8 also shows another form of control for oscillator i6. As has been described in the abovementioned Patent 2,250,511, the output frequency of a reex velocity modulation oscillator of the present type may be adjusted within narrow limits by adjusting the electron transit time by varying the potential of the reflector electrode 28. In the present instance, a version of the voltage of condenser 34 or 34 is also-applied to the reiiector electrode 28. Thus voltage divider 14 may bevprovided with a second output tap 11. The changes in the voltage appearing between tap 'il and ground are led through a condenser 18, and superposed on a, suitable negative or positive potential derived from a source 82 and voltage divider 83. The.combined voltage thus obtained is applied to reflector 28 after a delay and smoothing action provided by resistor 19 and condenser 8l. Adjustment of the voltage divider 83 will thereby adjust the center frequency of the range over which the rei-lector electrode can tune oscillator I6. The delay provided by circuit 19, 8l prevents the oscillator frequency from jumping narrow range of frequencies.

so far off as to cause the discriminator to lose control, while preserving a fairly wide range of frequency control by .the reector electrode.`

The voltage derived from condenser 34-34' serves to modify this reflector electrode voltage in one sense or another to change the tuning accordingly.

While the complete tuning control of oscillator I6 could be effected by this type of el' etrical control of the reflector electrode 28, in which case oscillator 12 or power tube 41 would be eliminated, it is. more desirable to utilize both the thermal tuning and the electrical tuning, since this produces a much-improved form of control. Thus,

as stated above, the frequency control bythe to immediately change the oscillator frequency by changing the voltage of the reflector 28 before the change in strut tuning current can become eiective to change the setting of .the thermal strut. `After a brief interval of time, during which the thermal strut expands to its new extension corresponding to the changed control voltage, the reector voltage returns to the value determined by voltage divider 83. This will be seen more clearly from the consideration that condenser 18 permits only changes in the control voltage to pass therethrough.

Accordingly, by this circuit the change in con- 14 controlled from the voltage of condenser 34 or 34'. One such type is illustrated in Fig. 9, which shows a thermally tuned Klystron oscillator of the type disclosed in Varian Patent No. 2,242,275, lissued May 20, 1941.

This tube has a cathode 8| which is generally maintained at a, high'negative potential with respect to ground by any suitable source of potential, schematically illustrated at 82. This source 82 causes a stream of electrons to be projected along the axis of the tube through a pair of grids 83 forming portions of the walls of a cavity resonator 84. As is well known, the ultra-high-frequency oscillating field between grids 83 velocity modulates the electrons of the beam, so that, during their subsequent travel through the field-free drift space formed by tubular member 86, the electrons become bunched or grouped. The grouped electrons then pass through a second pair of grids 81, forming portions of the walls of a second cavity resonator 88, to which they give up their ultrahigh frequency electro-magnetic energy. The resonators 84 and 88 may be coupled by a suitable coupling link 89 to produce sustained oscillations, which may be abstracted from resonator 86 by any suitable terminal device, such as the loop 9| and output transmission line 92.

The electrons after leaving the grids 81, are collected on the metallic walls of the evacuated housing, which may be conically shaped, as at 93, to provide a greater heat dissipatng area. Each of the resonators 84 and 88 is provided with a flexible diaphragm 94 and 96,' whereby the grids 83, 81 of the resonators may have their spacings ladjusted to produce a corresponding adjustment of the resonant frequencies of the resonators, and

l of the output frequency of the oscillator.

denser voltage is effective immediately to retune the oscillator, while the changed control voltage is effective to maintain' the oscillator retuned to its new frequency. By proper adjustment of the variable tap 11 of voltage divider 14, and of the magnitudes of the circuit constants and voltages utilized, the quick correction provided by varying the reflector electrode voltage can be made to just compensate for the thermal inertia delay inherent in the tuning strut, so that, by the combined thermal and electrical tuning of this modication, the frequency of oscillator i6 can be maintained lat the desired value substantially without lag, and vcan be made to respond quickly to changes in the intermediate frequency for a wide range of rates of change of this intermediate frequency.

If desired, the actual condenser voltage (or a portion thereof) may be applied directly to the reector electrode. Before lock-in, this will merely extend'the scanning range, because the variation in reflector voltage will'change the frequency in the same sense as the variation in s trut current. However, the reflector scanning should be small to permit proper oscillations over the entire thermaltuning range.

After lock-in, any drift will change the reflector voltage to change the frequency. This will ,Frequency adjustment is again provided by means of thermal tuning struts 91 and 98 having series-connected heater coils 99 and |0| for varying the separation between pairs of flanges |02, |03 and |03, |04, which are respectively rigidly connected to the corresponding pairs of grids 83 and 81. The grids 83 and 81 are normally urged together, either by the action of atmospheric pressure vupon the evacuated housing, or

'by tension springs |06 tending to draw the flanges-|02, |03 and |03, |04 together. Heating current passing through heater coils |0| and 99 serves to separate the grids 83 and 81 and thereby change the output frequency of the oscillator accordingly.

change the strut current, which will `tend to choose an average value at which the reflector voltage is correct.

The invention has thus far been described solely with respect to a thermally. tuned reiiex velocity modulation oscillator. It is to be understood that manyother types lofoscillator may also be Itvwill be clear that the oscillator of Fig. 9 may be utilized in place of that of Fig. 2 or` of Fig. 8 merely by connecting the heater coils 99 and |0| in place of the heater coil 33 of the preceding figures.

Several other types of oscillators may also be utilized. Another such type is shown in Fig. 10. which is similar to Fig. 9 except that the heater coils 99 and |0| and the struts 91 and 98 have been replaced by magnetically actuated-force transmitting means. Thus, xed to anges |02 and |03 is a pair of soft iron cores or pole pieces |01 and |08 having series-connected energizing coils |09, which are so connected as to pro duce opposite magnetic poles at the narrow gap ||2 between the pole pieces |08, |09. Pole pieces |01, |08 are valso connected by a flat spring H3, preferably also of magnetic material, which thereby serves to complete the magnetic circuit.

Accordingly, the passage. of current through grids |09 and will produce a force between the pole pieces corresponding to this wall. Since the pole pieces are rigidly fastened to the flanges y 15 |02, |03, a corresponding force is exerted upon the grids 83. This force is opposed by compression spring |06' and by spring II3 so that the displacement of grids 83 from their normal posiv tions willl be substantially proportional to the current Aflowing through grids |09 and III. A similar arrangement is provided for adjusting grids 81, the elements being designated by the same reference numerals, but primed.

Preferably, the device of Fig. 10 operates on direct current, and, accordingly, the voltage across condenser 36 or 34' may be applied directly tothe magnetic energizing coils |09, III, |09', I I I'. If necessary, a direct current amplifier may be interposed to produce a stronger energizing current. However, the apparatus of Fig. 10 may also be utilized on alternating current, butin this case, some smoothing action may be desirable which may be provided by suitable dashpots interposed between the flanges |02, |03 and |03, |08. If desired, the device of Fig. 10 could be substituted directly for the oscillator i6 of Fig. 2 or Fig. 8 by merely rectifying the current formerly l supplied to heater 33 and substituting coils |09, III for the heater 33. I

It will be noted that, for increasing tuning control current, the device of Fig. 10 will produce increasing output frequencies, which is opposite to the effect produced in Figs. 2, 8 or 9. For this reason, when using the device of Fig. 10 the discriminator output terminals should be interchanged, in order to produce stable operation.

Although the above invention has been described With respect to a radio pulse object detecting system, it is not necessarily restricted to such use. For example, the system may equally well be utilized to cause the oscillator I6 to :e-

main in step or to correspond in frequency, with any frequency source.

For example, in Fig. 11 the oscillator I6 may be A kept in fixed-frequency-difference relation to a continuous wave source of reference frequency -I I4, which may be either a standard oscillator,

IIS, whose conductivity is controlled from a suitable pulse generator I l1, which is adapted to producey a periodic sequence of pulses having a repetition frequency, for example, similar to that of pulse transmitter II. The transmission through amplifier IIB is adapted to thereby be intermit- I tently interrupted by the pulse wave derived from pulse generator vI I1, so that the output derived .from blocking amplifier I I6 will be of the same character as the input to the frequency discriminator I9 in Fig- 1.

Accordingly, this pulsed intermediate frequency wave may be fed to the discriminator I9 to actuate control circuit 2| for controlling oscillator I6 in exactly the same man-ner as in the preceding figures. In this way, the system already described may also be utilized to maintain oscillator I6 in synchronism with any suitable continuous wave source of oscillations It will be obvious that the present invention may also be utilized in many other instances.

` Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof. it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative quency condition of said device for modifying said control' voltage to interrupt said periodic frequency variation.

2. Frequency control apparatus forV an ultra.l

high frequency velocity modulation cavity resonator device having thermally extensible tuning means for varying theoperating frequency thereof and heater means for actuating said tuning means, comprising a source of electrical energy connected to said heater means, means for producing a periodically varying control voltage, means for controlling said source by said control voltage to correspondingly periodically vary the operating frequency ofsaid device, and means responsive to a predetermined relation between said operating frequency and a desired frequency condition of said device for modifying 'said control) voltage to interrupt said periodic frequency variation.

3. Apparatus as in claim lfurther comprising means responsive to said modified control volt-v age for maintaining said operating frequency in a desired condition.

4. Frequency control apparatus for a reex velocity modulation oscillator having thermallyactuated tuning means for varying the operating frequency thereof and a tuning control heater coil for heating said tuning means, comprising a source of energy for said heater coil, and a sawtooth oscillation generator for controlling the energy output of said source, said generator including a condenser and means for periodically charging and discharging said condenser to produce said saw-tooth oscillations, whereby the operating frequency of said oscillator is periodically scanned over a pre-determined frequency range corresponding to said saw-tooth oscillations.

5. Apparatus as in claim 3, further comprising means for interrupting said periodic scanning and for maintaining the operating frequency of said oscillator at a' predetermined value, said lastnamed means comprising means responsive to a predetermined relation between said operating frequency and said desired frequency for interrupting the discharging of said condenser, whereby the energization of said heater by said source is maintained substantially constant and said operating frequency is correspondingly maintained at the desired value.A

6. Automatic frequency control apparatus for a superheterodyne receiver for received pulsed high frequency waves and having a local oscillator and thermally-actuated tuning means for said oscillator, and also having heater means for actuating said tuning means, comprising frequency disef-ommen signal-repre ntingtrleviationfof,said local oscillater;frequencyi'romfai:desiredrrelaticn withreod`- cing a pulsedcontrol specttto-saidreceived wave, a source of heating 1 energy, 'sawtooth Vvoltage generator means for producing a sawtoothvoltage'wave, means for periodically varying the application of heating energy from 'said source to said heater means under the control of saidsawtooth voltage wave, wherebysaid localosclllator voltage is periodically scanned in frequency in correspondence with the aasgaas variation in said sawtooth voltage, and means responsive to said pulsed control wave for inter rupting the variation of said sawtoth wave upon the attainment of a local oscillator frequency' havingsaid desired relation with respect to said received wave.

7. Apparatus as in claim 6, wherein said sawtooth voltage generator means comprises a conf denser, means for slowly charging said condenser, and means for rapidly discharging said condenser, and wherein said interrupting vmeans comprises means responsive to the pulses of said pulsed control voltage for substantially preventing discharge of said condenser.

8. Apparatus as in claim 6, wherein said local oscillator comprises a cavity resonator, means for projecting an-electron stream through said resonator, and means including a reflector electrode for causing said electronsI to reenter said resonator, and said 'apparatus further including means for controlling the potential of said reflector electrode by said sawtooth voltage wave simultaneously with the control of heating energy to said heating means by said sawtooth voltage wave.

l 12. The method of operating a reflux oscillator Vhaving a tunable cavity resonator and a reector electrode, comprising producing a control voltage corresponding to 4a desired frequency condition of said oscillator, tuning said resonator in correspondence with said control voltage, and controlling the potential of said reflector'electrode in correspondence with the rate of change of said control voltage.

13. The method of operating an electron disy charge device having a pair of electron-permeable electrodesconnected to a tuned circuit, comprising the steps of projecting an electron stream through said electrodes, producing a control signal representing a, desired frequency condition of said device, varying the resonant. fres quency of said tuned circuit in Iaccordance with 9. Frequency control apparatus for a-reflex ves iocity modulation oscillator having a tunable cavity resonator and a rei-lector electrode, comprising means for producing a periodically varying control Voltage and means including a delay cir'- cuit for controlling the tuning pfsaid resonator in accordance with said control voltage and for controlling the potential of said reflector electrode in accordance with a delayed version of said control voltage, whereby to periodically vary the output frequency of saidosclllator over a predetermined frequency band.

10. Frequency control apparatus for a reflex `velocity modulation oscillator having a tunable cavity resonator and a reflector electrode, comprising means for producing a control Voltage for controlling the output frequency of said device, means for tuning said resonator by said control voltage, means including a delay circuit for deriving a time-delayed version of said control voltage, and means for controlling the potential of said reflector electrode in accordance with said time-delayed version of said'control voltage.

ll. High frequency apparatus comprising a cavity resonator having a pair of electron-persaid reector electrode corresponding toy said.

. time-delayed control voltage.

said signal, and varying the transit time of electrons through said device in accordance with the rate of change of said signal.

14. High frequency apparatus comprising a pair of electron-permeable electrodes defining agap therebetween, a tuned circuit connected to said electrodes, means for projecting a stream of electrons through said gap, means including a, reilectcr electrode for re-directing said electrons back through said gap, whereby oscillations may be sustained in said circuit, means for producing a control signal corresponding to a desired frequency of said oscillations, means for controlling the resonant frequency of said tuned circuit in accordance with said signal, and means for applying a potential to said reiiecto'r elcctrode corresponding to the rate of change of said signal.

15. Control apparatus for a high frequency apparatus having separate rapid-acting and slowacting tuning means, comprising means for producing a control signal corresponding to a desired frequency condition of said apparatus,

means for controlling said slow-acting tuning means in correspondence with said signal, and means for controlling said rapid-acting tuning means inresponse to the rate of change of said signal, l

16. High frequency apparatus comprising a cavity resonator having electron-permeable walls defining a gap therebetween. means for projecting an electron stream through said gap, means including a reflector electrode for re-directing said stream back through said gap, whereby osci1- lations are produced within said resonator, means including a thermally-extensible member' and heating means for said member for controlling the tuning of said resonator, means for producing a control signal corresponding to ,deviation of the frequency of said oscillations from a desired value. means for applying a control potential 'to said reflector electrode derived from said signal, and further means for energizing said heating means in accordance with .said signal.

17. Frequency control apparatus for a recx velocity-modulation oscillatorV having a cavity resonator and thermally-actuated means for tuning said resonator and also having a reflector electrode, comprising means'for producing a periodically varying control voltage, means for controlling the potential of said reflector electrode by said control voltage, andmeans for heating said thermally-actuated tuning means in correspondence with said varying control voltage, whereby the output frequencyA of said oscillator is periodically variedv over a predetermined frequency band.

18. The method of controlling a reflex velocitymodulation electron discharge device having a reflector electrode and a cavity resonator with thermally-actuated tuning means, comprising the steps of producing a control voltage for controlling the output of said device, supplying heating energy to said thermally-actuated tuning means in response to said control voltage, and simultaneously controlling the potential of said reflector electrode in response to said control voltage.

19. Frequency control apparatus for a high frequency device having thermally-actuated tuning control means and heater means for actuating said tuning control means, comprising in combination a source of oscillations having a frequency high in comparison to the thermal lag of said tuning means, means for applying said oscillations to said heater means to control said tuning means, means for producing a control voltage having an amplitude characteristic dependent upon the desired operating frequency of said device, and means for controlling said applying means by said control voltage to thereby control said operating frequency.

20. Frequency control apparatus for a highfrequency device having thermally actuated tuning means, comprising in combination a source of oscillations connected to energize said tuning means and having a frequency high in comparison to the thermal lag of said tuning means, and means responsive to deviations of the operating frequency of said device from a desired value for controllably adjusting the output of said source tol alter the operating frequency of said device to reduce the magnitude of said deviations.

21. Frequency control apparatus for a high frequency device having thermally-actuated tuning means and heater means for actuating said tuning means, comprising in combination a source of oscillations connected to energize said heater means and having a frequency high in comparison to the thermal lag of said tuning means, and means responsive to deviations of the operating frequency of said device from a desired value for controllably adjusting the output of said source so as to alter the operating frequency of said device to reduce the magnitude of said deviations.

22. Control apparatus for a cavity resonator electron discharge device having a cavity resonator with thermally-actuated tuning means therefor and electric heating means for energizing said tuning means to vary the resonant frequency of said resonator, comprising means for producing a control voltage corresponding to a desired condition of said device, means comprising a source of square wave voltage for energizing said heating means, and means for controlling said energizing means in response to said control voltage, whereby said device may be maintained at said desired condition.

23. Frequency control apparatus for a high frequency electron discharge device having thermally-actuated tuning means and heater means for actuating said tuning means, comprising, in combination, a source of square wave oscillations connected to energize said heater means, and means responsive to deviations of the operating frequency of said device from a desired value for adjusting the output of vsaid source to thereby alter the operating frequency of said device so as to reduce said deviations.

24. Frequency control apparatus for a thermally tuned high frequency device having thermally-actuated tuning means and heater means for actuating said tuning means, comprising, in combination, a source of oscillations connected to energize sai`d heating means, and means responsive to deviations of the operating frequency of said device from a desired value for controllably adjusting the output of said source so as to alter the operating frequency of said device to reduce the magnitude of said deviations.

25. Frequency control apparatus for a. thermally-tuned device having thermally-actuated tuning means and heater means for actuating said tuning means, comprising an oscillator coupled to supply power to said heater means and having a source of exciting potential, a control tube in series with said source, means for producing a control voltage corresponding to a ,desired frequency condition of said device, and means for controlling the conductivity of said control tube in response to said voltage, whereby the output of said oscillator is controlled to adjust the energization of said heater means and thereby the frequency of said device.

HORACE MYRL S'I'EARNS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name s Date 2,012,603 Fuchs Aug. 27, 1935 2,098,331 Bowman Nov. 9, 1937 2,326,737 Andrews Aug. 17, 1943 2,242,249 Varian May 20, 1941 2,242,275 Varian May 20, 1941 2 245,627 Varian June 17, 1941 v 2,162,335 Jacob June 13, 1939 2,203,750 Sherman June 1l, 1940 2,262,147 Owsley Nov. 11, 1941 1,847,160 Affel Mar. 1, 1932 2,294,942 Varian Sept. 8, 1942 2,287,925 White June 30, 1942 2,284,266 Bellescize May 26, 1942 2,254,601 Feld Sept. 2, 1941 2,374,810 Fremlin May 1, 1945 FOREIGN PATENTS Number Country Date 537,518 Great Britain June 25, 1941 Cmite of Correction Patent No. 2,434,293. January 13, 194s.

HORACE MYRL sTEARNs Y It is hereby certified that errors appear in the above numbered patent requiring correctionvasfvcilows: In the grant, lines 6 and 7, in the heading to the drawing, line 2, and in` the leading to `Vthe printed specification, lines2 and 3, title of invention, for Frequency Control 4of an YOsfillatore-of-thehrjelocity Modulation Type read Oscillator Frequency Control System; column 8, lines 67 `and 68, strike out the word positivef;"and thatthe said patent should be read with these corrections therein that the same may conform to the record of the casein the Patent Oce.

Signed and sealed this 4th day of May, A. D. 1948.

THOMAS F. MURPHY,

Assistant Uommz'asoner of Potente. 

